Within most of North America, telephone subscribers possess ten-digit telephone numbers of the form AAA-XXX-YYYY. The first three digits (AAA) represent an area code, designating the region in which the subscriber resides. The second three digits (XXX) represent an exchange, i.e., the local central office serving the subscriber. Finally, the last four digits (YYYY) represent a line number associated with the subscriber. For out-of-area code calls, the calling party dials the called party's ten-digit number. The calling party's call is received at the caller's Local Exchange Carrier (LEC) that, in turn, routes the call to an Inter-Exchange Carrier, such as AT&T. The IXC in turn, routes the call to the called party's LEC. A local call within the same exchange, or to a nearby exchange in the same Local Access and Transport Area (LATA), may not require dialing of the full ten-digits. Rather, for such calls, a caller simply dials the last seven digits of the called party's ten-digit number, hereinafter referred to as a "POTS" number. The manner in which ten-digit and seven digit calls are routed is well known.
Today, a person's telephone POTS number is often that person's most ubiquitous identifier. For that reason, individuals that move want to retain their prior seven-digit POTS number because others often identify them by their previous phone number. The ideal solution would be for subscribers to retain their originally assigned seven-digit POTS number for life, regardless of their present residence and the exchange providing service to that residence. However, this approach would require a data base inquiry for each call to perform any necessary translation for POTS numbers that might not correspond to a particular geographical area.
A present day solution to the problem of providing subscribers with lifetime numbers is to designate a Service Access Code (SAC), such as 500 or 700, to accommodate such numbers. Unfortunately, the lifetime numbers assigned to subscribers usually have no relationship to their present or past seven-digit POTS numbers. If subscribers were assigned lifetime numbers corresponding to their seven-digit POTS numbers, then conflicts would arise between subscribers in different areas having the same seven-digit POTS numbers. The inability of a subscriber to receive a lifetime number that corresponds to that subscriber's seven-digit POTS number has been an impediment to widespread adoption of this service.
One possible solution to this problem would be to assign different SACs to subscribers having the same seven-digit POTS number. Thus, for example two subscribers having the same seven-digit POTS number XXX-YYYY might be assigned a SAC of 400 and 500 respectively, to differentiate between them. The use of multiple SACs avoids ambiguity but destroys the correspondence relating a POTS number to a single lifetime number.
Thus, there is a need for a technique for routing calls to lifetime numbers that comprise a single SAC and the subscriber's seven-digit POTS number while avoiding a conflict between in routing calls to different subscribers having the same lifetime number (i.e., the same SAC and seven-digit POTS number).